1. Field of the Invention
The invention relates to a brake control system that applies braking forces to wheels.
2. Description of the Related Art
Recently, electronic brake control systems have been provided in vehicles. The brake control system controls braking forces for wheels so that the optimum braking forces are applied to the vehicle according to a situation in which the vehicle is traveling. In the brake control system, wheel cylinder pressures for the wheels are monitored using pressure sensors, and electromagnetic valves are controlled so that the wheel cylinder pressures are equal to target hydraulic pressures calculated based on the amount by which a driver depresses a brake pedal.
In the brake control system, pumps are used to increase the hydraulic pressures, and the electromagnetic valves are used to control the hydraulic pressures. The actuators, such as the pumps and the electromagnetic valves, operate using power from a battery that functions as a power supply. Therefore, it is necessary to examine how the brake control system should execute a brake control if a voltage decreases due to a malfunction of the battery.
Published Japanese Translation of PCT application No. 10-503446 (JP-A-10-503446) describes a circuit for a brake system that executes an electrical control of braking force distribution. In the circuit, the function for executing the electrical braking force distribution control is maintained until the voltage of a power supply falls below a predetermined limit value. In contrast, the function of the Antilock Brake System (ABS) and the function of the Traction Control System (TCS) are suppressed.
In the technology described in Published Japanese Translation of PCT application No. 10-503446, a control is executed to suppress the function of the ABS and the function of the TCS when the voltage of the power supply decreases. Thus, the function of the ABS and the function of the TCS are stopped when the function of the ABS and the function of the TCS are still usable. Also, if a control is executed to stop operations of actuators in the brake control system in a predetermined sequence, the control is complicated, resulting in an increase in cost. U.S. Pat. No. 5,795,039 A1 mentions that during construction of a brake system it should be considered that control methods may fail if the supplied voltage is lowered. According to this teaching the same actuators “only ” perform an electronic brake force distribution, but no longer perform an ABS/TCS, if a failure of a sensor is detected. U.S. Pat. No. 5 795 039 A1 furthermore discloses to monitor the brake system including the supply voltage, the sensors, and so on. When a malfunction of any of the components is detected, the monitoring circuits will intervene to bring the system in a “safe” condition.